When an infrared-guided missile attacks a ground target, natural or human factors can cause the infrared characteristics of the target to weaken or even disappear. The seeker cannot or intermittently detects the target, which significantly affects the guidance accuracy. To solve this problem, a digital twin guidance law is proposed for hitting a weak infrared target on the ground. On the basis of an infrared seeker in the physical world, digital twin models of the target and guidance laws are developed in the digital world. The state parameters of the missile motion and control in the guidance process at each point in time are obtained by simulation, and saved as the digital twin of the guidance process. During guidance process, when the seeker cannot obtain the measurement signal, its digital twin is activated immediately to take over and provide the control system with the acceleration order. Simulations show that the digital twin of the seeker can provide the missile control system with the maneuver order to accurately guide the missile when the infrared seeker is unable to capture the signal. The digital twin guidance law is robust against infrared camouflage, interference, and bad weather, and has broad application prospects.